1. Field of the Invention
The present invention relates to a rotary-head digital magnetic recording and reproducing apparatus that digitally records video and audio signals on magnetic tape, and in particular, to a fast-speed playback technique for the same.
2. Description of Related Art
Rotary-head digital magnetic recording and reproducing apparatus of various formats have been announced heretofore. As a typical example of such apparatus, a professional digital VTR for broadcasting use, employing a system known as the D-2 format, will be described below.
FIG. 1 is a simplified block diagram showing an example of a digital VTR employing the D-2 format. In FIG. 1, the reference numeral 201 is a video signal input terminal, 202 is an audio signal input terminal, 203 and 204 are A/D converters, 205 is a digital recording signal processor, 207 is a digital modulation processor, 208 and 209 are recording amplifiers, 210 and 211 are record/playback selecting switches, 212 and 213 are head selecting switches, 214, 215, 216, and 217 are record/playback heads, 218 and 219 are playback amplifiers, 220 is a digital demodulation processor, 221 is a digital reproducing signal processor, 224 and 225 are D/A converters, 226 is a video signal output terminal, and 227 is an audio signal output terminal.
FIG. 2 shows the track format for recording on tape employed in the D-2 format digital VTR. As shown in FIG. 2, in the D-2 format, a cue track, a time code track, and a control track are recorded along the longitudinal direction of the magnetic tape, while the video and audio signals are digitally recorded on tracks that are produced diagonally across the magnetic tape. Two channels of audio signals are recorded on each side of the video signal, so that there are a total of four audio channels recorded per track.
The operation of the above VTR will now be described with reference to FIG. 1. The composite video signal input at the input terminal 201 is sampled by the A/D converter 203 at a frequency four times the subcarrier frequency, i.e., at 14.318 MHz, for quantization into eight-bit words. On the other hand, the audio signal input at the input terminal 202 is sampled at 48 kHz, and quantized into 20-bit words, using the A/D converter 204. For simplicity, only one channel of audio signal input is shown in FIG. 1, but actually, there are four audio input channels. The digitized video and four-channel audio signals are supplied to the digital recording signal processor 205. The digital recording signal processor 205 processes the video and four-channel audio signals with respect to the time axis in accordance with the format and appends error-correcting codes to them. The error-correcting codes are appended to the video and four-channel audio signals independently of one another. In the digital modulation processor 207, digital modulation processing is performed on the signals in accordance with a prescribed modulation scheme. The signals output from the digital modulation processor 207 are fed to the recording amplifiers 208 and 209, passed through the record/playback selecting switches 210 and 211, and distributed via the head selecting switches 212 and 213 over the record/playback heads 214, 215, 216, and 217 for recording onto the magnetic tape in accordance with the track format shown in FIG. 2. In the above format, the data rate after error-correcting coding is 127M bits/sec. and the video signals for one field are recorded on six separate tracks.
Reproduction of the signals is performed in the following manner. The signals reproduced by the respective heads 214, 215, 216, and 217 are fed, via the head selecting switches 212, 213 and the record/playback selecting switches 210, 211, to the playback amplifiers 218, 219 which amplify the reproduced signals before supplying them to the digital demodulation processor 220. The digital signals demodulated by the digital demodulation processor 220 are passed to the digital reproducing signal processor 221 which performs error-correcting decoding and other processing to decode the digital signals into a data train of ordinary video and four-channel audio signals for output. The signals output from the digital reproducing signal processor 221 are converted by the D/A converters 224 and 225 back into the original video and four-channel audio signals.
FIG. 3 shows a format of error-correcting codes proposed by Ken Onishi, Takashi Itow, Hirofumi Nishikawa, Kazuhiro Sugiyama, Hideo Yoshida, Masato Nagasawa, Kihei Ido, Kunihiko Nakagawa, Yoshinobu Ishida, and Satoshi Kunii, in "An Experimental Home-Use Digital VCR with Three Dimensional DCT and Superimposed Error Correction Coding," IEEE Trans. Consumer Electronics, vol. 37, no. 3, pp. 252-260, August 1991 (Reference 1). According to this format, the video signal V is first encoded with an (N, K, d.sub.1) code to produce horizontal error-correcting codewords C1, and then encoded with an (L+Q, L, d.sub.2) to produce vertical error-correcting codewords C2. In an (n, k, d) code, n denotes the codeword length, k the information length, and d the distance between codewords. S designates a superimposing code; additional information of m.times. Ls is vertically encoded with an (Ls+Qs, Ls, d.sub.3) code (in general Ls&lt;L, Qs&gt;Q) and superimposed on the check symbol area C1 to obtain a total of (Ls+Qs).times.N codewords. The additional information can thus be appended to the ordinary information area as necessary.
A method of using punctured coding instead of superimposed coding is disclosed by Hideo Yoshida, Takahiko Nakamura, Atsuhiro Yamagishi, Tohru Inoue, and Ken Onishi, in "A Note on Error Correcting Code Structure For Home Use Digital VTR using Punctured Codes" Institute of Electronics, Information and Communication Engineers, Technical Research Report IT91-15, May 14, 1991 (Reference 2). With this method also, additional information can be appended as necessary. It is also discussed in the report that when decoding using the above method, punctured portions of information are regarded as erasures and erasure correction is performed to reconstruct the information. That is, error correction is performed using the horizontal codewords C1, and errors (such as long burst errors) that cannot be corrected with the C1 codewords are corrected as erasures.
Accordingly, the professional digital VTR of the above construction offers such features as high reliability, high image quality, high sound quality, and sophisticated editing functions that are needed for professional use. The above digital VTR, however, has had the problem that degradation of image quality is caused in fast speed playback mode in which dynamic tracking following (DTF) is not performed. The coding format shown in FIG. 3 involves the double encoding of information, i.e. in both horizontal and vertical directions, and therefore, when applied to digital VTRs, the code offers a powerful error-correcting capability and ensures sufficiently high reliability in normal playback mode. However, in fast speed playback mode not using DTF, since the heads are moved in such a way to traverse the tracks, it is only possible to reproduce part of the C1 code produced along the track direction, and therefore, it is necessary to devise a means for reproducing the information by making the maximum use of that part of the code.
There has also been the problem that efficient error-correction coding is not possible for auxiliary data that is usually not necessary but that becomes necessary on occasions.